Ambient air may contain Mercury (Hg), including gas-phase mercury-compounds. Gas-phase mercury compounds may include gaseous elemental mercury (GEM) or a gaseous mercury compound(s) (GMC), such as Mercury Bromide (HgBr2), Mercury Chloride (HgCl2), Mercury Oxide (HgO), Mercury Sulfate (HgSO4), Mercury Nitrite (Hg(NO2)2), Mercury Nitrate (Hg(NO3)2), Mercury Iodide (HgI2), or Mercury Fluoride (HgF2). Other GMC types may also be present in ambient air.
About 1 m3 to about 10 m3 of ambient air may contain up to a few hundred picograms (10−12 grams) of GEM or GMC. At these low concentrations, typical gaseous mercury detection systems, including gaseous mercury detection systems that include gas-chromatography mass spectrometers (GCMS), can be ineffective in accurately measuring GEM or GMC that may be present in ambient air.
Additionally, typical gaseous mercury detection systems do not measure GMC without decomposing through high-temperature treatment the GMC, i.e., the actual compounds, into their constituent components. Conventional GEM detection systems capture GEM or GMC from ambient air into a mercury collector and then release a high temperatures GEM or the elemental constituents of decomposed GMC into a detection device such as a cold vapor atomic fluorescence spectrometer. A conventional mercury detection system, however, does not measure a GMC as a compound, but as its constituent elements.
FIG. 1 is a schematic diagram of a prior-art GEM detection system 100. The GEM detection system 100 includes a mercury collector 102 (e.g., denuder) fluidly coupled to a mercury detector 104. Typically, ambient air flows through the mercury collector 102. The mercury collector 102 includes a collection surface 103 configured to capture GMC from the ambient air. The collection surface 103 may include a potassium chloride (KCl) coating. After collecting GMC, the mercury collector 104 and, in particular, the collection surface 103 is heated, using a heater 106, to a high temperature in an attempt to completely release or desorb all the GMC from the collection surface 103. Such a temperature may be in excess of about 500° C.
At those high temperature, e.g., temperatures around or above 500° C., most GMC will decompose into GEM and their elemental constituents. For example, a published decomposition temperature for Mercury Sulfate (HgSO4) is 500° C. See Kurt H. Stern, High Temperature Properties and Thermal Decomposition of Inorganic Salts with Oxyanions, 21 Sep. 2000, p 66. Similarly, Mercury Nitrite (Hg(NO2)2) begins to decompose at a temperature as low as 50° C. and readily decomposes at 90° C. into Mercury Oxide (HgO) and Dinitrogen Trioxide (N2O3). See Id at 146. Mercury Nitrate (Hg(NO3)2) decomposes into Mercury Oxide (HgO) appreciably at 160° C. See Id at 147. Mercury Oxide (HgO) decomposes into GEM at under 400° C. G. See Van Praagh, Physical Chemistry, 1950, pp 267-268. Most GMC decomposes over a range of temperatures, but generally speaking, the higher the temperature, the faster the decomposition rate.
After heating the collection surface 103, the GEM or the elemental constituents of decomposed GMC released from the collection surface 103 then pass into the detector 104 to be measured. However, because the collection surface has been heated to temperatures in excess of the decomposition temperature of most GMC, the GMC will have decomposed into their elemental constituents, making the detector 104 unable to accurately detect concentrations of the GMC as compounds initially collected by the collection surface 103.
Accordingly, users and designers of gaseous mercury detection systems continue to seek improved detection systems.